33 Introduction to Programming Using Java (Inglés) autor David J Eck

Vista previa del material en texto

Introduction to Programming Using Java
Version 5.0, December 2006
(Version 5.0.2, with minor corrections, November 2007)
David J. Eck
Hobart and William Smith Colleges
ii
c©1996–2007, David J. Eck
David J. Eck (eck@hws.edu)
Department of Mathematics and Computer Science
Hobart and William Smith Colleges
Geneva, NY 14456
This book can be distributed in unmodified form with no restrictions.
Modified versions can be made and distributed provided they are distributed
under the same license as the original. More specifically: This work is
licensed under the Creative Commons Attribution-Share Alike 2.5 License.
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sa/2.5/ or send a letter to Creative Commons, 543 Howard Street, 5th
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The web site for this book is: http://math.hws.edu/javanotes
Contents
Preface xiii
1 The Mental Landscape 1
1.1 Machine Language . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Asynchronous Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 The Java Virtual Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.4 Building Blocks of Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
1.5 Object-oriented Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.6 The Modern User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
1.7 The Internet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Quiz on Chapter 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2 Names and Things 19
2.1 The Basic Java Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
2.2 Variables and Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.2.1 Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.2.2 Types and Literals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.2.3 Variables in Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.3 Objects and Subroutines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2.3.1 Built-in Subroutines and Functions . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 Operations on Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.3.3 Introduction to Enums . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4 Text Input and Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
2.4.1 A First Text Input Example . . . . . . . . . . . . . . . . . . . . . . . . . . 37
2.4.2 Text Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.4.3 TextIO Input Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
2.4.4 Formatted Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
2.4.5 Introduction to File I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
2.5 Details of Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
2.5.1 Arithmetic Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
2.5.2 Increment and Decrement . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.5.3 Relational Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.5.4 Boolean Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
2.5.5 Conditional Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
2.5.6 Assignment Operators and Type-Casts . . . . . . . . . . . . . . . . . . . . 48
2.5.7 Type Conversion of Strings . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.5.8 Precedence Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
2.6 Programming Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
iii
iv CONTENTS
2.6.1 Java Development Kit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.6.2 Command Line Environment . . . . . . . . . . . . . . . . . . . . . . . . . 52
2.6.3 IDEs and Eclipse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.6.4 The Problem of Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Exercises for Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
Quiz on Chapter 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
3 Control 61
3.1 Blocks, Loops, and Branches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.1.1 Blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
3.1.2 The Basic While Loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.1.3 The Basic If Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.2 Algorithm Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
3.2.1 Pseudocode and Stepwise Refinement . . . . . . . . . . . . . . . . . . . . 66
3.2.2 The 3N+1 Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.2.3 Coding, Testing, Debugging . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.3 while and do..while . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
3.3.1 The while Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
3.3.2 The do..while Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.3.3 break and continue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
3.4 The for Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.4.1 For Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3.4.2 Example: Counting Divisors . . . . . . . . . . . . . . . . . . . . . . . . . . 83
3.4.3 Nested for Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
3.4.4 Enums and for-each Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
3.5 The if Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89
3.5.1 The Dangling else Problem . . . . . . . . . . . . . . . . . . . . . . . . . . 89
3.5.2 The if...else if Construction . . . . . . . . . . . . . . . . . . . . . . . . . . 89
3.5.3 If Statement Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.5.4 The Empty Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
3.6 The switch Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96
3.6.1 The Basic switch Statement . . . . . . . . . . . . . . . . . . . . . . . . . . 96
3.6.2 Menus and switch Statements . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.6.3 Enums in switch Statements . . . . . . . . . . . . . . . . . . . . . . . . . 98
3.6.4 Definite Assignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
3.7 Exceptions and try..catch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.7.1 Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
3.7.2 try..catch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.7.3 Exceptions in TextIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
3.8 GUI Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
Exercises for Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Quiz on Chapter 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
4 Subroutines 117
4.1 Black Boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
4.2 Static Subroutines and Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.2.1 Subroutine Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119
4.2.2 Calling Subroutines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
CONTENTS v4.2.3 Subroutines in Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
4.2.4 Member Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
4.3 Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.3.1 Using Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127
4.3.2 Formal and Actual Parameters . . . . . . . . . . . . . . . . . . . . . . . . 128
4.3.3 Overloading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.3.4 Subroutine Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
4.3.5 Throwing Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132
4.3.6 Global and Local Variables . . . . . . . . . . . . . . . . . . . . . . . . . . 133
4.4 Return Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
4.4.1 The return statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
4.4.2 Function Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
4.4.3 3N+1 Revisited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
4.5 APIs, Packages, and Javadoc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.5.1 Toolboxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.5.2 Java’s Standard Packages . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.5.3 Using Classes from Packages . . . . . . . . . . . . . . . . . . . . . . . . . 142
4.5.4 Javadoc . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
4.6 More on Program Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
4.6.1 Preconditions and Postconditions . . . . . . . . . . . . . . . . . . . . . . . 146
4.6.2 A Design Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
4.6.3 The Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
4.7 The Truth About Declarations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
4.7.1 Initialization in Declarations . . . . . . . . . . . . . . . . . . . . . . . . . 154
4.7.2 Named Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
4.7.3 Naming and Scope Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
Exercises for Chapter 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Quiz on Chapter 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164
5 Objects and Classes 165
5.1 Objects and Instance Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165
5.1.1 Objects, Classes, and Instances . . . . . . . . . . . . . . . . . . . . . . . . 166
5.1.2 Fundamentals of Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
5.1.3 Getters and Setters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172
5.2 Constructors and Object Initialization . . . . . . . . . . . . . . . . . . . . . . . . 173
5.2.1 Initializing Instance Variables . . . . . . . . . . . . . . . . . . . . . . . . . 173
5.2.2 Constructors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
5.2.3 Garbage Collection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
5.3 Programming with Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179
5.3.1 Some Built-in Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180
5.3.2 Wrapper Classes and Autoboxing . . . . . . . . . . . . . . . . . . . . . . . 181
5.3.3 The class “Object” . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
5.3.4 Object-oriented Analysis and Design . . . . . . . . . . . . . . . . . . . . . 183
5.4 Programming Example: Card, Hand, Deck . . . . . . . . . . . . . . . . . . . . . . 185
5.4.1 Designing the classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185
5.4.2 The Card Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187
5.4.3 Example: A Simple Card Game . . . . . . . . . . . . . . . . . . . . . . . . 191
vi CONTENTS
5.5 Inheritance and Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194
5.5.1 Extending Existing Classes . . . . . . . . . . . . . . . . . . . . . . . . . . 194
5.5.2 Inheritance and Class Hierarchy . . . . . . . . . . . . . . . . . . . . . . . 196
5.5.3 Example: Vehicles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197
5.5.4 Polymorphism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
5.5.5 Abstract Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202
5.6 this and super . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
5.6.1 The Special Variable this . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
5.6.2 The Special Variable super . . . . . . . . . . . . . . . . . . . . . . . . . . 206
5.6.3 Constructors in Subclasses . . . . . . . . . . . . . . . . . . . . . . . . . . 208
5.7 Interfaces, Nested Classes, and Other Details . . . . . . . . . . . . . . . . . . . . 209
5.7.1 Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
5.7.2 Nested Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
5.7.3 Anonymous Inner Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . 214
5.7.4 Mixing Static and Non-static . . . . . . . . . . . . . . . . . . . . . . . . . 214
5.7.5 Static Import . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
5.7.6 Enums as Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 217
Exercises for Chapter 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 220
Quiz on Chapter 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223
6 Introduction to GUI Programming 225
6.1 The Basic GUI Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
6.1.1 JFrame and JPanel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
6.1.2 Components and Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229
6.1.3 Events and Listeners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 230
6.2 Applets and HTML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
6.2.1 JApplet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231
6.2.2 Reusing Your JPanels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233
6.2.3 Basic HTML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235
6.2.4 Applets on Web Pages . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 238
6.3 Graphics and Painting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 240
6.3.1 Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242
6.3.2 Colors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243
6.3.3 Fonts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244
6.3.4 Shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245
6.3.5 Graphics2D . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
6.3.6 An Example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
6.4 Mouse Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251
6.4.1 Event Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 252
6.4.2 MouseEvent and MouseListener . . . . . . . . . . . . . . . . . . . . . . . . 253
6.4.3 Mouse Coordinates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256
6.4.4 MouseMotionListeners and Dragging . . . . . . . . . . . . . . . . . . . . . 258
6.4.5 Anonymous Event Handlers . . . . . . . . . . . . . . . . . . . . . . . . . . 262
6.5 Timer and Keyboard Events . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . 264
6.5.1 Timers and Animation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264
6.5.2 Keyboard Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 266
6.5.3 Focus Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269
CONTENTS vii
6.5.4 State Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 270
6.6 Basic Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273
6.6.1 JButton . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275
6.6.2 JLabel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 276
6.6.3 JCheckBox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277
6.6.4 JTextField and JTextArea . . . . . . . . . . . . . . . . . . . . . . . . . . . 278
6.6.5 JComboBox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279
6.6.6 JSlider . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280
6.7 Basic Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 282
6.7.1 Basic Layout Managers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283
6.7.2 Borders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285
6.7.3 SliderAndComboBoxDemo . . . . . . . . . . . . . . . . . . . . . . . . . . 287
6.7.4 A Simple Calculator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 289
6.7.5 Using a null Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291
6.7.6 A Little Card Game . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293
6.8 Menus and Dialogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
6.8.1 Menus and Menubars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
6.8.2 Dialogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 300
6.8.3 Fine Points of Frames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
6.8.4 Creating Jar Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
Exercises for Chapter 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 306
Quiz on Chapter 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311
7 Arrays 313
7.1 Creating and Using Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313
7.1.1 Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314
7.1.2 Using Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314
7.1.3 Array Initialization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316
7.2 Programming With Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
7.2.1 Arrays and for Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 318
7.2.2 Arrays and for-each Loops . . . . . . . . . . . . . . . . . . . . . . . . . . . 320
7.2.3 Array Types in Subroutines . . . . . . . . . . . . . . . . . . . . . . . . . . 321
7.2.4 Random Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 322
7.2.5 Arrays of Objects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 324
7.2.6 Variable Arity Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
7.3 Dynamic Arrays and ArrayLists . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
7.3.1 Partially Full Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
7.3.2 Dynamic Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332
7.3.3 ArrrayLists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 335
7.3.4 Parameterized Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 339
7.3.5 Vectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342
7.4 Searching and Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
7.4.1 Searching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
7.4.2 Association Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 345
7.4.3 Insertion Sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
7.4.4 Selection Sort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 349
7.4.5 Unsorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 351
viii CONTENTS
7.5 Multi-dimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
7.5.1 Creating Two-dimensional Arrays . . . . . . . . . . . . . . . . . . . . . . 352
7.5.2 Using Two-dimensional Arrays . . . . . . . . . . . . . . . . . . . . . . . . 354
7.5.3 Example: Checkers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357
Exercises for Chapter 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 364
Quiz on Chapter 7 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370
8 Correctness and Robustness 373
8.1 Introduction to Correctness and Robustness . . . . . . . . . . . . . . . . . . . . . 373
8.1.1 Horror Stories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 374
8.1.2 Java to the Rescue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
8.1.3 Problems Remain in Java . . . . . . . . . . . . . . . . . . . . . . . . . . . 377
8.2 Writing Correct Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 378
8.2.1 Provably Correct Programs . . . . . . . . . . . . . . . . . . . . . . . . . . 378
8.2.2 Robust Handling of Input . . . . . . . . . . . . . . . . . . . . . . . . . . . 381
8.3 Exceptions and try..catch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 385
8.3.1 Exceptions and Exception Classes . . . . . . . . . . . . . . . . . . . . . . 386
8.3.2 The try Statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 388
8.3.3 Throwing Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 390
8.3.4 Mandatory Exception Handling . . . . . . . . . . . . . . . . . . . . . . . . 392
8.3.5 Programming with Exceptions . . . . . . . . . . . . . . . . . . . . . . . . 393
8.4 Assertions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396
8.5 Introduction to Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 399
8.5.1 Creating and Running Threads . . . . . . . . . . . . . . . . . . . . . . . . 400
8.5.2 Operations on Threads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 404
8.5.3 Mutual Exclusion with “synchronized” . . . . . . . . . . . . . . . . . . . . 405
8.5.4 Wait and Notify . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
8.5.5 Volatile Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 412
8.6 Analysis of Algorithms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413
Exercises for Chapter 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419
Quiz on Chapter 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424
9 Linked Data Structures and Recursion 427
9.1 Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 427
9.1.1 Recursive Binary Search . . . . . . . . . . . . . . . . . . . . . . . . . . . . 428
9.1.2 Towers of Hanoi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430
9.1.3 A Recursive Sorting Algorithm . . . . . . . . . . . . . . . . . . . . . . . . 432
9.1.4 Blob Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 435
9.2 Linked Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439
9.2.1 Recursive Linking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 439
9.2.2 Linked Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 441
9.2.3 Basic Linked ListProcessing . . . . . . . . . . . . . . . . . . . . . . . . . 441
9.2.4 Inserting into a Linked List . . . . . . . . . . . . . . . . . . . . . . . . . . 445
9.2.5 Deleting from a Linked List . . . . . . . . . . . . . . . . . . . . . . . . . . 447
9.3 Stacks, Queues, and ADTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 448
9.3.1 Stacks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 449
9.3.2 Queues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 452
9.3.3 Postfix Expressions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 456
CONTENTS ix
9.4 Binary Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 459
9.4.1 Tree Traversal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 460
9.4.2 Binary Sort Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 462
9.4.3 Expression Trees . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467
9.5 A Simple Recursive Descent Parser . . . . . . . . . . . . . . . . . . . . . . . . . . 470
9.5.1 Backus-Naur Form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 470
9.5.2 Recursive Descent Parsing . . . . . . . . . . . . . . . . . . . . . . . . . . . 472
9.5.3 Building an Expression Tree . . . . . . . . . . . . . . . . . . . . . . . . . . 476
Exercises for Chapter 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 479
Quiz on Chapter 9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 482
10 Generic Programming and Collection Classes 485
10.1 Generic Programming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 485
10.1.1 Generic Programming in Smalltalk . . . . . . . . . . . . . . . . . . . . . . 486
10.1.2 Generic Programming in C++ . . . . . . . . . . . . . . . . . . . . . . . . 487
10.1.3 Generic Programming in Java . . . . . . . . . . . . . . . . . . . . . . . . . 488
10.1.4 The Java Collection Framework . . . . . . . . . . . . . . . . . . . . . . . . 489
10.1.5 Iterators and for-each Loops . . . . . . . . . . . . . . . . . . . . . . . . . . 491
10.1.6 Equality and Comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . 492
10.1.7 Generics and Wrapper Classes . . . . . . . . . . . . . . . . . . . . . . . . 495
10.2 Lists and Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
10.2.1 ArrayList and LinkedList . . . . . . . . . . . . . . . . . . . . . . . . . . . 496
10.2.2 Sorting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499
10.2.3 TreeSet and HashSet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500
10.2.4 EnumSet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503
10.3 Maps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 504
10.3.1 The Map Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
10.3.2 Views, SubSets, and SubMaps . . . . . . . . . . . . . . . . . . . . . . . . 506
10.3.3 Hash Tables and Hash Codes . . . . . . . . . . . . . . . . . . . . . . . . . 509
10.4 Programming with the Collection Framework . . . . . . . . . . . . . . . . . . . . 511
10.4.1 Symbol Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
10.4.2 Sets Inside a Map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 512
10.4.3 Using a Comparator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 515
10.4.4 Word Counting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 517
10.5 Writing Generic Classes and Methods . . . . . . . . . . . . . . . . . . . . . . . . 519
10.5.1 Simple Generic Classes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 520
10.5.2 Simple Generic Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 521
10.5.3 Type Wildcards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 523
10.5.4 Bounded Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 527
Exercises for Chapter 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 531
Quiz on Chapter 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 535
11 Files and Networking 537
11.1 Streams, Readers, and Writers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537
11.1.1 Character and Byte Streams . . . . . . . . . . . . . . . . . . . . . . . . . 537
11.1.2 PrintWriter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 539
11.1.3 Data Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 540
11.1.4 Reading Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 541
x CONTENTS
11.1.5 The Scanner Class . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544
11.1.6 Serialized Object I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 545
11.2 Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 546
11.2.1 Reading and Writing Files . . . . . . . . . . . . . . . . . . . . . . . . . . . 546
11.2.2 Files and Directories . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 549
11.2.3 File Dialog Boxes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 552
11.3 Programming With Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 554
11.3.1 Copying a File . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 555
11.3.2 Persistent Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 557
11.3.3 Files in GUI Programs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 559
11.3.4 Storing Objects in Files . . . . . . . . . . . . . . . . . . . . . . . . . . . . 561
11.4 Networking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 568
11.4.1 URLs and URLConnections . . . . . . . . . . . . . . . . . . . . . . . . . . 569
11.4.2 TCP/IP and Client/Server . . . . . . . . . . . . . . . . . . . . . . . . . . 571
11.4.3 Sockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 572
11.4.4 A Trivial Client/Server . . . . . . . . . . . . . . . . . . . . . . . . . . . . 574
11.4.5 A Simple Network Chat . . . . . . . . . . . . . . . . . . . . . . . . . . . . 578
11.5 Network Programming and Threads . . . . . . . . . . . . . . . . . . . . . . . . . 581
11.5.1 A Threaded GUI Chat Program. . . . . . . . . . . . . . . . . . . . . . . . 582
11.5.2 A Multithreaded Server . . . . . . . . . . . . . . . . . . . . . . . . . . . . 585
11.5.3 Distributed Computing . . . . . . . . . . . . . . . . . . . . . . . . . . . . 588
11.6 A Brief Introduction to XML . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596
11.6.1 Basic XML Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 596
11.6.2 XMLEncoder and XMLDecoder . . . . . . . . . . . . . . . . . . . . . . . 598
11.6.3 Working With the DOM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 600
Exercises for Chapter 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 606
Quiz on Chapter 11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 609
12 Advanced GUI Programming 611
12.1 Images and Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 611
12.1.1 Images and BufferedImages . . . . . . . . . . . . . . . . . . . . . . . . . . 611
12.1.2 Working With Pixels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 617
12.1.3 Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 620
12.1.4 Cursors and Icons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 621
12.1.5 Image File I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 622
12.2 Fancier Graphics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 624
12.2.1 Measuring Text . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . 625
12.2.2 Transparency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 627
12.2.3 Antialiasing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 629
12.2.4 Strokes and Paints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 630
12.2.5 Transforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 633
12.3 Actions and Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 636
12.3.1 Action and AbstractAction . . . . . . . . . . . . . . . . . . . . . . . . . . 636
12.3.2 Icons on Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 638
12.3.3 Radio Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 639
12.3.4 Toolbars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 642
12.3.5 Keyboard Accelerators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 643
CONTENTS xi
12.3.6 HTML on Buttons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 645
12.4 Complex Components and MVC . . . . . . . . . . . . . . . . . . . . . . . . . . . 646
12.4.1 Model-View-Controller . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 646
12.4.2 Lists and ListModels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 647
12.4.3 Tables and TableModels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 650
12.4.4 Documents and Editors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 654
12.4.5 Custom Components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 655
12.5 Finishing Touches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660
12.5.1 The Mandelbrot Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 660
12.5.2 Design of the Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 662
12.5.3 Internationalization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 664
12.5.4 Events, Events, Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 666
12.5.5 Custom Dialogs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 668
12.5.6 Preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 669
Exercises for Chapter 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 671
Quiz on Chapter 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 673
Appendix: Source Files 675
xii CONTENTS
Preface
Introduction to Programming Using Java is a free introductory computer programming
textbook that uses Java as the language of instruction. It is suitable for use in an introductory
programming course and for people who are trying to learn programming on their own. There
are no prerequisites beyond a general familiarity with the ideas of computers and programs.
There is enough material for a full year of college-level programming. Chapters 1 through 7
can be used as a textbook in a one-semester college-level course or in a year-long high school
course.
This version of the book covers “Java 5.0”, and many of the examples use features that were
not present in earlier versions of Java. (Sometimes, you will see this version of Java referred
to as Java 1.5 instead of Java 5.0.) Note that Java applets appear throughout the pages of the
on-line version of this book. Many of those applets will be non-functional in Web browsers that
do not support Java 5.0.
The home web site for this book is http://math.hws.edu/javanotes/. The page at that
address contains links for downloading a copy of the web site and for downloading a PDF
version of the book.
∗ ∗ ∗
In style, this is a textbook rather than a tutorial. That is, it concentrates on explaining
concepts rather than giving step-by-step how-to-do-it guides. I have tried to use a conversational
writing style that might be closer to classroom lecture than to a typical textbook. You’ll
find programming exercises at the end of most chapters, and you will find a detailed solution
for each exercise, with the sort of discussion that I would give if I presented the solution in
class. (Solutions to the exercises can be found in the on-line version only.) I strongly advise
that you read the exercise solutions if you want to get the most out of this book. This is
certainly not a Java reference book, and it is not even close to a comprehensive survey of all
the features of Java. It is not written as a quick introduction to Java for people who already
know another programming language. Instead, it is directed mainly towards people who are
learning programming for the first time, and it is as much about general programming concepts
as it is about Java in particular. I believe that Introduction to Programming using Java is
fully competitive with the conventionally published, printed programming textbooks that are
available on the market. (Well, all right, I’ll confess that I think it’s better.)
There are several approaches to teaching Java. One approach uses graphical user interface
programming from the very beginning. Some people believe that object oriented programming
should also be emphasized from the very beginning. This is not the approach that I take. The
approach that I favor starts with the more basic building blocks of programming and builds
from there. After an introductory chapter, I cover procedural programming in Chapters 2, 3,
and 4. Object-oriented programming is introduced in Chapter 5. Chapters 6 covers the closely
related topic of event-oriented programming and graphical user interfaces. Arrays are covered in
Chapter 7. Chapter 8 marks a turning point in the book, moving beyond the fundamental ideas
xiii
xiv Preface
of programming to cover more advanced topics. Chapter 8 is mostly about writing robust and
correct programs, but it also has a section on parallel processing and threads. Chapters 9 and
10 cover recursion and data structures, including the Java Collection Framework. Chapter 11 is
about files and networking. Finally, Chapter 12 returns to the topic of graphical user interface
programming to cover some of Java’s more advanced capabilities.
∗ ∗ ∗
Major changes have been made in the fifth edition. Perhaps the most significant change is
the use of parameterized types in the chapter on generic programming. Parameterized types—
Java’s version of templates—were the most eagerly anticipated new feature in Java 5.0.
Other new features in Java 5.0 are also covered. Enumerated types are introduced, although
they are not covered in their full complexity. The “for-each” loop is covered and is used
extensively. Formatted output is also used extensively, and the Scanner class is covered (though
not until Chapter 11). Static import is covered briefly, as are variable arity methods.
The non-standard TextIO class that I use for input in the first half of the book has been
rewritten to support formatted output. I have also added some file I/O capabilities to this class
to make it possible to cover some examples that use files early in the book.
Javadoc comments are covered for the first time in this edition. Almost all code examples
have been revised to use Javadoc-style comments.
The coverage of graphical user interface programming has been reorganized, much of it has
been rewritten, and new material has been added. In previous editions, I emphasized applets.
Stand-alone GUI applications were covered at the end, almost as an afterthought. In the fifth
edition, the emphasis on applets is gone, and almost all examples are presented as stand-alone
applications. However, applet versions of each example are still presented on the web pages of
the on-line version of the book. The chapter on advanced GUI programming has been moved
to the end, and a significant amount of new material has been added, including coverage of
some of the features of Graphics2D.
Aside from the changes in content, the appearance of the book has been improved, especially
the appearance of the PDF version. For the firsttime, the quality of the PDF approaches that
of conventional textbooks.
∗ ∗ ∗
The latest complete edition of Introduction to Programming using Java is always available
on line at http://math.hws.edu/javanotes/. The first version of the book was written in 1996,
and there have been several editions since then. All editions are archived at the following Web
addresses:
• First edition: http://math.hws.edu/eck/cs124/javanotes1/ (Covers Java 1.0.)
• Second edition: http://math.hws.edu/eck/cs124/javanotes2/ (Covers Java 1.1.)
• Third edition: http://math.hws.edu/eck/cs124/javanotes3/ (Covers Java 1.1.)
• Fourth edition: http://math.hws.edu/eck/cs124/javanotes4/ (Covers Java 1.4.)
• Fifth edition: http://math.hws.edu/eck/cs124/javanotes5/ (Covers Java 5.0.)
Introduction to Programming using Java is free, but it is not in the public domain. As
of Version 5.0, it is published under the terms of the Creative Commons Attribution-Share
Alike 2.5 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-
sa/2.5/ or send a letter to Creative Commons, 543 Howard Street, 5th Floor, San Francisco,
California, 94105, USA. This license allows redistribution and modification under certain terms.
For example, you can:
Preface xv
• Post an unmodified copy of the on-line version on your own Web site (including the parts
that list the author and state the license under which it is distributed!).
• Give away or sell printed, unmodified copies of this book, as long as they meet the re-
quirements of the license.
• Make modified copies of the complete book or parts of it and post them on the web or
otherwise distribute them, provided that attribution to the author is given, the modifica-
tions are clearly noted, and the modified copies are distributed under the same license as
the original. This includes translations to other languages.
While it is not actually required by the license, I do appreciate hearing from people who
are using or distributing my work.
∗ ∗ ∗
A technical note on production: The on-line and PDF versions of this book are created
from a single source, which is written largely in XML. To produce the PDF version, the XML
is processed into a form that can be used by the TeX typesetting program. In addition to XML
files, the source includes DTDs, XSLT transformations, Java source code files, image files, a
TeX macro file, and a couple of scripts that are used in processing. I have not made the source
materials available for download, since they are not in a clean enough form to be publishable,
and because it would require a fair amount of expertise to make any use of them. However,
they are not meant to be secret, and I am willing to make them available on request.
∗ ∗ ∗
Professor David J. Eck
Department of Mathematics and Computer Science
Hobart and William Smith Colleges
Geneva, New York 14456, USA
Email: eck@hws.edu
WWW: http://math.hws.edu/eck/
xvi Preface
Chapter 1
Overview: The Mental Landscape
When you begin a journey, it’s a good idea to have a mental map of the terrain you’ll
be passing through. The same is true for an intellectual journey, such as learning to write
computer programs. In this case, you’ll need to know the basics of what computers are and
how they work. You’ll want to have some idea of what a computer program is and how one is
created. Since you will be writing programs in the Java programming language, you’ll want to
know something about that language in particular and about the modern, networked computing
environment for which Java is designed.
As you read this chapter, don’t worry if you can’t understand everything in detail. (In fact,
it would be impossible for you to learn all the details from the brief expositions in this chapter.)
Concentrate on learning enough about the big ideas to orient yourself, in preparation for the
rest of the book. Most of what is covered in this chapter will be covered in much greater detail
later in the book.
1.1 The Fetch and Execute Cycle: Machine Language
A computer is a complex system consisting of many different components. But at the
heart—or the brain, if you want—of the computer is a single component that does the actual
computing. This is the Central Processing Unit , or CPU. In a modern desktop computer,
the CPU is a single “chip” on the order of one square inch in size. The job of the CPU is to
execute programs.
A program is simply a list of unambiguous instructions meant to be followed mechanically
by a computer. A computer is built to carry out instructions that are written in a very simple
type of language called machine language . Each type of computer has its own machine
language, and the computer can directly execute a program only if the program is expressed in
that language. (It can execute programs written in other languages if they are first translated
into machine language.)
When the CPU executes a program, that program is stored in the computer’s main mem-
ory (also called the RAM or random access memory). In addition to the program, memory
can also hold data that is being used or processed by the program. Main memory consists of a
sequence of locations. These locations are numbered, and the sequence number of a location
is called its address. An address provides a way of picking out one particular piece of informa-
tion from among the millions stored in memory. When the CPU needs to access the program
instruction or data in a particular location, it sends the address of that information as a sig-
nal to the memory; the memory responds by sending back the data contained in the specified
1
2 CHAPTER 1. THE MENTAL LANDSCAPE
location. The CPU can also store information in memory by specifying the information to be
stored and the address of the location where it is to be stored.
On the level of machine language, the operation of the CPU is fairly straightforward (al-
though it is very complicated in detail). The CPU executes a program that is stored as a
sequence of machine language instructions in main memory. It does this by repeatedly reading,
or fetching , an instruction from memory and then carrying out, or executing , that instruc-
tion. This process—fetch an instruction, execute it, fetch another instruction, execute it, and so
on forever—is called the fetch-and-execute cycle . With one exception, which will be covered
in the next section, this is all that the CPU ever does.
The details of the fetch-and-execute cycle are not terribly important, but there are a few
basic things you should know. The CPU contains a few internal registers, which are small
memory units capable of holding a single number or machine language instruction. The CPU
uses one of these registers—the program counter , or PC—to keep track of where it is in the
program it is executing. The PC stores the address of the next instruction that the CPU should
execute. At the beginning of each fetch-and-execute cycle, the CPU checks the PC to see which
instruction it should fetch. During the course of the fetch-and-execute cycle, the number in the
PC is updated to indicate the instruction that is to be executed in the next cycle. (Usually,
but not always, this is just the instruction that sequentially follows the current instruction in
the program.)
∗ ∗ ∗
A computer executes machine language programs mechanically—that is without under-
standing them or thinking about them—simply because of the way it is physically put together.
This is not an easy concept. A computer is a machine built of millions of tiny switches called
transistors, which have the property that they can be wired together in such a way that an
output from one switch can turn another switch on or off. As a computer computes, these
switches turn each other on or off in a pattern determined both by the way they are wired
together and by the program that the computer is executing.
Machine language instructions are expressed as binary numbers. A binary number is made
up of just two possible digits, zero and one. So, a machine languageinstruction is just a sequence
of zeros and ones. Each particular sequence encodes some particular instruction. The data that
the computer manipulates is also encoded as binary numbers. A computer can work directly
with binary numbers because switches can readily represent such numbers: Turn the switch on
to represent a one; turn it off to represent a zero. Machine language instructions are stored
in memory as patterns of switches turned on or off. When a machine language instruction
is loaded into the CPU, all that happens is that certain switches are turned on or off in the
pattern that encodes that particular instruction. The CPU is built to respond to this pattern
by executing the instruction it encodes; it does this simply because of the way all the other
switches in the CPU are wired together.
So, you should understand this much about how computers work: Main memory holds
machine language programs and data. These are encoded as binary numbers. The CPU fetches
machine language instructions from memory one after another and executes them. It does
this mechanically, without thinking about or understanding what it does—and therefore the
program it executes must be perfect, complete in all details, and unambiguous because the CPU
can do nothing but execute it exactly as written. Here is a schematic view of this first-stage
understanding of the computer:
1.2. ASYNCHRONOUS EVENTS 3
Data to memory
Data from memory
Address for
reading/writing
data
1011100001
Program
counter:
CPU
Memory
(Location 0)
(Location 1)
(Location 2)
(Location 3)
(Location 10)
00101110
11010011
01010011
00010000
10111111
10100110
11101001
00000111
10100110
00010001
00111110
1.2 Asynchronous Events: Polling Loops and Interrupts
The CPU spends almost all of its time fetching instructions from memory and executing
them. However, the CPU and main memory are only two out of many components in a real
computer system. A complete system contains other devices such as:
• A hard disk for storing programs and data files. (Note that main memory holds only a
comparatively small amount of information, and holds it only as long as the power is turned
on. A hard disk is necessary for permanent storage of larger amounts of information, but
programs have to be loaded from disk into main memory before they can actually be
executed.)
• A keyboard and mouse for user input.
• A monitor and printer which can be used to display the computer’s output.
• A modem that allows the computer to communicate with other computers over telephone
lines.
• A network interface that allows the computer to communicate with other computers
that are connected to it on a network.
• A scanner that converts images into coded binary numbers that can be stored and
manipulated on the computer.
The list of devices is entirely open ended, and computer systems are built so that they can
easily be expanded by adding new devices. Somehow the CPU has to communicate with and
control all these devices. The CPU can only do this by executing machine language instructions
(which is all it can do, period). The way this works is that for each device in a system, there
is a device driver , which consists of software that the CPU executes when it has to deal
with the device. Installing a new device on a system generally has two steps: plugging the
device physically into the computer, and installing the device driver software. Without the
device driver, the actual physical device would be useless, since the CPU would not be able to
communicate with it.
4 CHAPTER 1. THE MENTAL LANDSCAPE
∗ ∗ ∗
A computer system consisting of many devices is typically organized by connecting those
devices to one or more busses. A bus is a set of wires that carry various sorts of information
between the devices connected to those wires. The wires carry data, addresses, and control
signals. An address directs the data to a particular device and perhaps to a particular register
or location within that device. Control signals can be used, for example, by one device to alert
another that data is available for it on the data bus. A fairly simple computer system might
be organized like this:
Input/
Output
Controller
Data bus
Address bus
Control bus
CPU Empty Slot
for future
Expansion
Keyboard Network
Interface
......
Network Cable
Video
Controller
and
Monitor
Memory
Now, devices such as keyboard, mouse, and network interface can produce input that needs
to be processed by the CPU. How does the CPU know that the data is there? One simple idea,
which turns out to be not very satisfactory, is for the CPU to keep checking for incoming data
over and over. Whenever it finds data, it processes it. This method is called polling , since
the CPU polls the input devices continually to see whether they have any input data to report.
Unfortunately, although polling is very simple, it is also very inefficient. The CPU can waste
an awful lot of time just waiting for input.
To avoid this inefficiency, interrupts are often used instead of polling. An interrupt is
a signal sent by another device to the CPU. The CPU responds to an interrupt signal by
putting aside whatever it is doing in order to respond to the interrupt. Once it has handled
the interrupt, it returns to what it was doing before the interrupt occurred. For example, when
you press a key on your computer keyboard, a keyboard interrupt is sent to the CPU. The
CPU responds to this signal by interrupting what it is doing, reading the key that you pressed,
processing it, and then returning to the task it was performing before you pressed the key.
Again, you should understand that this is a purely mechanical process: A device signals an
interrupt simply by turning on a wire. The CPU is built so that when that wire is turned on,
the CPU saves enough information about what it is currently doing so that it can return to
the same state later. This information consists of the contents of important internal registers
such as the program counter. Then the CPU jumps to some predetermined memory location
and begins executing the instructions stored there. Those instructions make up an interrupt
handler that does the processing necessary to respond to the interrupt. (This interrupt handler
is part of the device driver software for the device that signalled the interrupt.) At the end of
1.2. ASYNCHRONOUS EVENTS 5
the interrupt handler is an instruction that tells the CPU to jump back to what it was doing;
it does that by restoring its previously saved state.
Interrupts allow the CPU to deal with asynchronous events. In the regular fetch-and-
execute cycle, things happen in a predetermined order; everything that happens is “synchro-
nized” with everything else. Interrupts make it possible for the CPU to deal efficiently with
events that happen “asynchronously,” that is, at unpredictable times.
As another example of how interrupts are used, consider what happens when the CPU needs
to access data that is stored on the hard disk. The CPU can access data directly only if it is
in main memory. Data on the disk has to be copied into memory before it can be accessed.
Unfortunately, on the scale of speed at which the CPU operates, the disk drive is extremely
slow. When the CPU needs data from the disk, it sends a signal to the disk drive telling it
to locate the data and get it ready. (This signal is sent synchronously, under the control of a
regular program.) Then, instead of just waiting the long and unpredictalble amount of time
that the disk drive will take to do this, the CPU goes on with some other task. When the disk
drive has the data ready, it sends an interrupt signal to the CPU. The interrupt handler can
then read the requested data.
∗ ∗ ∗
Now, you might have noticed that all this only makes sense if the CPU actually has several
tasks to perform. If it has nothing better to do, it might as well spend its time polling for input
or waiting for disk drive operations to complete. All modern computers use multitaskingto
perform several tasks at once. Some computers can be used by several people at once. Since the
CPU is so fast, it can quickly switch its attention from one user to another, devoting a fraction
of a second to each user in turn. This application of multitasking is called timesharing . But a
modern personal computer with just a single user also uses multitasking. For example, the user
might be typing a paper while a clock is continuously displaying the time and a file is being
downloaded over the network.
Each of the individual tasks that the CPU is working on is called a thread . (Or a process;
there are technical differences between threads and processes, but they are not important here.)
At any given time, only one thread can actually be executed by a CPU. The CPU will continue
running the same thread until one of several things happens:
• The thread might voluntarily yield control, to give other threads a chance to run.
• The thread might have to wait for some asynchronous event to occur. For example, the
thread might request some data from the disk drive, or it might wait for the user to press
a key. While it is waiting, the thread is said to be blocked , and other threads have a
chance to run. When the event occurs, an interrupt will “wake up” the thread so that it
can continue running.
• The thread might use up its allotted slice of time and be suspended to allow other threads
to run. Not all computers can “forcibly” suspend a thread in this way; those that can
are said to use preemptive multitasking . To do preemptive multitasking, a computer
needs a special timer device that generates an interrupt at regular intervals, such as 100
times per second. When a timer interrupt occurs, the CPU has a chance to switch from
one thread to another, whether the thread that is currently running likes it or not.
Ordinary users, and indeed ordinary programmers, have no need to deal with interrupts and
interrupt handlers. They can concentrate on the different tasks or threads that they want the
computer to perform; the details of how the computer manages to get all those tasks done are
not important to them. In fact, most users, and many programmers, can ignore threads and
6 CHAPTER 1. THE MENTAL LANDSCAPE
multitasking altogether. However, threads have become increasingly important as computers
have become more powerful and as they have begun to make more use of multitasking. Indeed,
threads are built into the Java programming language as a fundamental programming concept.
Just as important in Java and in modern programming in general is the basic concept of
asynchronous events. While programmers don’t actually deal with interrupts directly, they
do often find themselves writing event handlers, which, like interrupt handlers, are called
asynchronously when specified events occur. Such “event-driven programming” has a very
different feel from the more traditional straight-through, synchronous programming. We will
begin with the more traditional type of programming, which is still used for programming
individual tasks, but we will return to threads and events later in the text.
∗ ∗ ∗
By the way, the software that does all the interrupt handling and the communication with
the user and with hardware devices is called the operating system . The operating system is
the basic, essential software without which a computer would not be able to function. Other
programs, such as word processors and World Wide Web browsers, are dependent upon the
operating system. Common operating systems include Linux, DOS, Windows 2000, Windows
XP, and the Macintosh OS.
1.3 The Java Virtual Machine
Machine language consists of very simple instructions that can be executed directly by
the CPU of a computer. Almost all programs, though, are written in high-level programming
languages such as Java, Pascal, or C++. A program written in a high-level language cannot
be run directly on any computer. First, it has to be translated into machine language. This
translation can be done by a program called a compiler . A compiler takes a high-level-language
program and translates it into an executable machine-language program. Once the translation
is done, the machine-language program can be run any number of times, but of course it can only
be run on one type of computer (since each type of computer has its own individual machine
language). If the program is to run on another type of computer it has to be re-translated,
using a different compiler, into the appropriate machine language.
There is an alternative to compiling a high-level language program. Instead of using a
compiler, which translates the program all at once, you can use an interpreter , which translates
it instruction-by-instruction, as necessary. An interpreter is a program that acts much like a
CPU, with a kind of fetch-and-execute cycle. In order to execute a program, the interpreter
runs in a loop in which it repeatedly reads one instruction from the program, decides what is
necessary to carry out that instruction, and then performs the appropriate machine-language
commands to do so.
One use of interpreters is to execute high-level language programs. For example, the pro-
gramming language Lisp is usually executed by an interpreter rather than a compiler. However,
interpreters have another purpose: they can let you use a machine-language program meant for
one type of computer on a completely different type of computer. For example, there is a pro-
gram called “Virtual PC” that runs on Macintosh computers. Virtual PC is an interpreter that
executes machine-language programs written for IBM-PC-clone computers. If you run Virtual
PC on your Macintosh, you can run any PC program, including programs written for Windows.
(Unfortunately, a PC program will run much more slowly than it would on an actual IBM clone.
The problem is that Virtual PC executes several Macintosh machine-language instructions for
1.3. THE JAVA VIRTUAL MACHINE 7
each PC machine-language instruction in the program it is interpreting. Compiled programs
are inherently faster than interpreted programs.)
∗ ∗ ∗
The designers of Java chose to use a combination of compilation and interpretation. Pro-
grams written in Java are compiled into machine language, but it is a machine language for
a computer that doesn’t really exist. This so-called “virtual” computer is known as the Java
virtual machine. The machine language for the Java virtual machine is called Java byte-
code . There is no reason why Java bytecode could not be used as the machine language of a
real computer, rather than a virtual computer.
However, one of the main selling points of Java is that it can actually be used on any
computer. All that the computer needs is an interpreter for Java bytecode. Such an interpreter
simulates the Java virtual machine in the same way that Virtual PC simulates a PC computer.
Of course, a different Jave bytecode interpreter is needed for each type of computer, but
once a computer has a Java bytecode interpreter, it can run any Java bytecode program. And
the same Java bytecode program can be run on any computer that has such an interpreter.
This is one of the essential features of Java: the same compiled program can be run on many
different types of computers.
J a v aP r o g r a m C o m p i l e r J a v aB y t e c o d eP r o g r a m
J a v a I n t e r p r e t e rf o r M a c O SJ a v a I n t e r p r e t e rf o r W i n d o w sJ a v a I n t e r p r e t e rf o r L i n u x
Why, you might wonder, use the intermediate Java bytecode at all? Why not just distribute
the original Java program and let each person compile it into the machine language of whatever
computer they want to run it on? There are many reasons. First of all, a compiler has to
understand Java, a complex high-level language. The compiler is itself a complex program. A
Java bytecode interpreter, on the other hand, is a fairly small, simple program. This makes it
easy to write a bytecode interpreter for a new type of computer; once that is done,that computer
can run any compiled Java program. It would be much harder to write a Java compiler for the
same computer.
Furthermore, many Java programs are meant to be downloaded over a network. This leads
to obvious security concerns: you don’t want to download and run a program that will damage
your computer or your files. The bytecode interpreter acts as a buffer between you and the
program you download. You are really running the interpreter, which runs the downloaded
program indirectly. The interpreter can protect you from potentially dangerous actions on the
part of that program.
I should note that there is no necessary connection between Java and Java bytecode. A pro-
gram written in Java could certainly be compiled into the machine language of a real computer.
And programs written in other languages could be compiled into Java bytecode. However, it is
the combination of Java and Java bytecode that is platform-independent, secure, and network-
compatible while allowing you to program in a modern high-level object-oriented language.
8 CHAPTER 1. THE MENTAL LANDSCAPE
∗ ∗ ∗
I should also note that the really hard part of platform-independence is providing a “Graph-
ical User Interface”—with windows, buttons, etc.—that will work on all the platforms that
support Java. You’ll see more about this problem in Section 1.6.
1.4 Fundamental Building Blocks of Programs
There are two basic aspects of programming: data and instructions. To work with
data, you need to understand variables and types; to work with instructions, you need to
understand control structures and subroutines. You’ll spend a large part of the course
becoming familiar with these concepts.
A variable is just a memory location (or several locations treated as a unit) that has been
given a name so that it can be easily referred to and used in a program. The programmer only
has to worry about the name; it is the compiler’s responsibility to keep track of the memory
location. The programmer does need to keep in mind that the name refers to a kind of “box”
in memory that can hold data, even if the programmer doesn’t have to know where in memory
that box is located.
In Java and most other languages, a variable has a type that indicates what sort of data
it can hold. One type of variable might hold integers—whole numbers such as 3, -7, and 0—
while another holds floating point numbers—numbers with decimal points such as 3.14, -2.7,
or 17.0. (Yes, the computer does make a distinction between the integer 17 and the floating-
point number 17.0; they actually look quite different inside the computer.) There could also
be types for individual characters (’A’, ’;’, etc.), strings (“Hello”, “A string can include many
characters”, etc.), and less common types such as dates, colors, sounds, or any other type of
data that a program might need to store.
Programming languages always have commands for getting data into and out of variables
and for doing computations with data. For example, the following “assignment statement,”
which might appear in a Java program, tells the computer to take the number stored in the
variable named “principal”, multiply that number by 0.07, and then store the result in the
variable named “interest”:
interest = principal * 0.07;
There are also “input commands” for getting data from the user or from files on the computer’s
disks and “output commands” for sending data in the other direction.
These basic commands—for moving data from place to place and for performing
computations—are the building blocks for all programs. These building blocks are combined
into complex programs using control structures and subroutines.
∗ ∗ ∗
A program is a sequence of instructions. In the ordinary “flow of control,” the computer
executes the instructions in the sequence in which they appear, one after the other. However,
this is obviously very limited: the computer would soon run out of instructions to execute.
Control structures are special instructions that can change the flow of control. There are
two basic types of control structure: loops, which allow a sequence of instructions to be repeated
over and over, and branches, which allow the computer to decide between two or more different
courses of action by testing conditions that occur as the program is running.
For example, it might be that if the value of the variable “principal” is greater than 10000,
then the “interest” should be computed by multiplying the principal by 0.05; if not, then the
1.5. OBJECT-ORIENTED PROGRAMMING 9
interest should be computed by multiplying the principal by 0.04. A program needs some
way of expressing this type of decision. In Java, it could be expressed using the following “if
statement”:
if (principal > 10000)
interest = principal * 0.05;
else
interest = principal * 0.04;
(Don’t worry about the details for now. Just remember that the computer can test a condition
and decide what to do next on the basis of that test.)
Loops are used when the same task has to be performed more than once. For example,
if you want to print out a mailing label for each name on a mailing list, you might say, “Get
the first name and address and print the label; get the second name and address and print
the label; get the third name and address and print the label—” But this quickly becomes
ridiculous—and might not work at all if you don’t know in advance how many names there are.
What you would like to say is something like “While there are more names to process, get the
next name and address, and print the label.” A loop can be used in a program to express such
repetition.
∗ ∗ ∗
Large programs are so complex that it would be almost impossible to write them if there
were not some way to break them up into manageable “chunks.” Subroutines provide one way to
do this. A subroutine consists of the instructions for performing some task, grouped together
as a unit and given a name. That name can then be used as a substitute for the whole set of
instructions. For example, suppose that one of the tasks that your program needs to perform
is to draw a house on the screen. You can take the necessary instructions, make them into
a subroutine, and give that subroutine some appropriate name—say, “drawHouse()”. Then
anyplace in your program where you need to draw a house, you can do so with the single
command:
drawHouse();
This will have the same effect as repeating all the house-drawing instructions in each place.
The advantage here is not just that you save typing. Organizing your program into sub-
routines also helps you organize your thinking and your program design effort. While writing
the house-drawing subroutine, you can concentrate on the problem of drawing a house without
worrying for the moment about the rest of the program. And once the subroutine is written,
you can forget about the details of drawing houses—that problem is solved, since you have a
subroutine to do it for you. A subroutine becomes just like a built-in part of the language which
you can use without thinking about the details of what goes on “inside” the subroutine.
∗ ∗ ∗
Variables, types, loops, branches, and subroutines are the basis of what might be called
“traditional programming.” However, as programs become larger, additional structure is needed
to help deal with their complexity. One of the most effective tools that has been found is object-
oriented programming, which is discussed in the next section.
1.5 Objects and Object-oriented Programming
Programs must be designed. No one can just sit down at the computer and compose a
program of any complexity. The discipline called software engineering is concerned with
10 CHAPTER 1. THE MENTAL LANDSCAPE
the construction of correct, working, well-written programs. The software engineer tends to
use accepted and proven methods for analyzing the problem to be solved and for designing a
program to solve that problem.
During the 1970s and into the 80s, the primary software engineering methodology was
structured programming .The structured programming approach to program design was
based on the following advice: To solve a large problem, break the problem into several pieces
and work on each piece separately; to solve each piece, treat it as a new problem which can itself
be broken down into smaller problems; eventually, you will work your way down to problems
that can be solved directly, without further decomposition. This approach is called top-down
programming .
There is nothing wrong with top-down programming. It is a valuable and often-used ap-
proach to problem-solving. However, it is incomplete. For one thing, it deals almost entirely
with producing the instructions necessary to solve a problem. But as time went on, people
realized that the design of the data structures for a program was as least as important as the
design of subroutines and control structures. Top-down programming doesn’t give adequate
consideration to the data that the program manipulates.
Another problem with strict top-down programming is that it makes it difficult to reuse
work done for other projects. By starting with a particular problem and subdividing it into
convenient pieces, top-down programming tends to produce a design that is unique to that
problem. It is unlikely that you will be able to take a large chunk of programming from another
program and fit it into your project, at least not without extensive modification. Producing
high-quality programs is difficult and expensive, so programmers and the people who employ
them are always eager to reuse past work.
∗ ∗ ∗
So, in practice, top-down design is often combined with bottom-up design . In bottom-up
design, the approach is to start “at the bottom,” with problems that you already know how to
solve (and for which you might already have a reusable software component at hand). From
there, you can work upwards towards a solution to the overall problem.
The reusable components should be as “modular” as possible. A module is a component of a
larger system that interacts with the rest of the system in a simple, well-defined, straightforward
manner. The idea is that a module can be “plugged into” a system. The details of what goes on
inside the module are not important to the system as a whole, as long as the module fulfills its
assigned role correctly. This is called information hiding , and it is one of the most important
principles of software engineering.
One common format for software modules is to contain some data, along with some sub-
routines for manipulating that data. For example, a mailing-list module might contain a list of
names and addresses along with a subroutine for adding a new name, a subroutine for printing
mailing labels, and so forth. In such modules, the data itself is often hidden inside the module;
a program that uses the module can then manipulate the data only indirectly, by calling the
subroutines provided by the module. This protects the data, since it can only be manipulated
in known, well-defined ways. And it makes it easier for programs to use the module, since they
don’t have to worry about the details of how the data is represented. Information about the
representation of the data is hidden.
Modules that could support this kind of information-hiding became common in program-
ming languages in the early 1980s. Since then, a more advanced form of the same idea has
more or less taken over software engineering. This latest approach is called object-oriented
programming , often abbreviated as OOP.
1.5. OBJECT-ORIENTED PROGRAMMING 11
The central concept of object-oriented programming is the object , which is a kind of module
containing data and subroutines. The point-of-view in OOP is that an object is a kind of self-
sufficient entity that has an internal state (the data it contains) and that can respond to
messages (calls to its subroutines). A mailing list object, for example, has a state consisting
of a list of names and addresses. If you send it a message telling it to add a name, it will
respond by modifying its state to reflect the change. If you send it a message telling it to print
itself, it will respond by printing out its list of names and addresses.
The OOP approach to software engineering is to start by identifying the objects involved in
a problem and the messages that those objects should respond to. The program that results is
a collection of objects, each with its own data and its own set of responsibilities. The objects
interact by sending messages to each other. There is not much “top-down” in such a program,
and people used to more traditional programs can have a hard time getting used to OOP.
However, people who use OOP would claim that object-oriented programs tend to be better
models of the way the world itself works, and that they are therefore easier to write, easier to
understand, and more likely to be correct.
∗ ∗ ∗
You should think of objects as “knowing” how to respond to certain messages. Different
objects might respond to the same message in different ways. For example, a “print” message
would produce very different results, depending on the object it is sent to. This property of
objects—that different objects can respond to the same message in different ways—is called
polymorphism .
It is common for objects to bear a kind of “family resemblance” to one another. Objects
that contain the same type of data and that respond to the same messages in the same way
belong to the same class. (In actual programming, the class is primary; that is, a class is
created and then one or more objects are created using that class as a template.) But objects
can be similar without being in exactly the same class.
For example, consider a drawing program that lets the user draw lines, rectangles, ovals,
polygons, and curves on the screen. In the program, each visible object on the screen could be
represented by a software object in the program. There would be five classes of objects in the
program, one for each type of visible object that can be drawn. All the lines would belong to
one class, all the rectangles to another class, and so on. These classes are obviously related;
all of them represent “drawable objects.” They would, for example, all presumably be able to
respond to a “draw yourself” message. Another level of grouping, based on the data needed
to represent each type of object, is less obvious, but would be very useful in a program: We
can group polygons and curves together as “multipoint objects,” while lines, rectangles, and
ovals are “two-point objects.” (A line is determined by its endpoints, a rectangle by two of its
corners, and an oval by two corners of the rectangle that contains it.) We could diagram these
relationships as follows:
12 CHAPTER 1. THE MENTAL LANDSCAPED r a w a b l e O b j e c tM u l t i p o i n t O b j e c t T w o P o i n t O b j e c tP o l y g o n C u r v e L i n e R e c t a n g l e O v a l
DrawableObject, MultipointObject, and TwoPointObject would be classes in the program.
MultipointObject and TwoPointObject would be subclasses of DrawableObject. The class
Line would be a subclass of TwoPointObject and (indirectly) of DrawableObject. A subclass of
a class is said to inherit the properties of that class. The subclass can add to its inheritance and
it can even “override” part of that inheritance (by defining a different response to some method).
Nevertheless, lines, rectangles, and so on are drawable objects, and the class DrawableObject
expresses this relationship.
Inheritance is a powerful means for organizing a program. It is also related to the problem
of reusing software components. A class is the ultimate reusable component. Not only can it
be reused directly if it fits exactly into a program you are trying to write, but if it just almost
fits, you can still reuse it by defining a subclass and making only the small changes necessary
to adapt it exactly to your needs.
So, OOP is meant to be both a superior program-development tool and a partial solution
to the software reuse problem. Objects,